Scripting Question

gchilders · 10-07-2008, 09:27 PM

Hi all,
I'm stymied by an anomaly I find when using multiple parameters and variables in a script. For example:

ls -l /dir1/dir2/dir3/{file1,file2,file3}

will produce the proper results:

-rw-r--r-- 1 jdoe users 0 Oct 7 14:34 /dir1/dir2/dir3/file1
-rw-r--r-- 1 jdoe users 0 Oct 7 14:34 /dir1/dir2/dir3/file2
-rw-r--r-- 1 jdoe users 0 Oct 7 14:34 /dir1/dir2/dir3/file3

However, if I use variable substitution to execute the same command:

DIR=/dir1/dir2/dir3
FILES="file1,file2,file3"
ls -l $DIR/{$FILES}

will produce:

/bin/ls: /dir1/dir2/dir3/{file1,file2,file3}: No such file or directory

What do I need to do to successfully use variable substitution?

Thanks,

Kenhelm · 10-07-2008, 11:57 PM

Try

Code:

DIR=/dir1/dir2/dir3
FILES="file1,file2,file3"
ls -l $(eval echo $DIR/{$FILES})

Code:

eval echo $DIR/{$FILES}
/dir1/dir2/dir3/file1 /dir1/dir2/dir3/file2 /dir1/dir2/dir3/file3

Mr. C. · 10-08-2008, 12:03 AM

The problem is that brace expansion occurs before variable expansion:

Code:

$ man bash:

EXPANSION
   ...
       The order of expansions is: brace expansion, tilde  expansion,  parame-
       ter,  variable  and arithmetic expansion and command substitution (done
       in a left-to-right fashion), word splitting, and pathname expansion.

Inside of the braces, when brace expansion occurs, there is the string of characters "$FILES", and this matches no file names. What remains is:

Code:

ls -l $DIR/$FILE

which is dutifully variable expanded:

Code:

ls -l /dir1/dir2/dir3/file1,file2,file3

and of course you see the consequences. In these situations, eval is your friend:

Code:

eval ls -l $DIR/$FILE

gchilders · 10-08-2008, 08:31 AM

Thanks, that's exactly what I needed.

My actual original command was:

scp $SOURCEDIR/{$FILES} $HOST:$TARGETDIR

which does not work, even though:

echo "scp $SOURCEDIR/{$FILES} $HOST:$TARGETDIR"

printed out the command exactly as I wanted to see it.

But the correct format:

scp $(eval echo $SOURCEDIR/{$FILES}) $HOST:$TARGETDIR

now works correctly in the script.

Many thanks! I don't think I would have figured this out on my own.

Regards,